This invention relates to forming metal shapes. More particularly, it relates to forming those metals, alloys, and composites that are identified as superplastic.
A superplastic metal or alloy is one which, under certain conditions, exhibits a very low resistance to deformation and a high plasticity. Such metals can be deformed to an extraordinary degree without failure as compared to conventional metals or alloys, and increasing attention is being paid to superplastic alloys and methods of forming them.
One of the earliest patents to describe superplastic forming of metals is Fields, Jr. et al U.S. Pat. No. 3,340,101. This patent describes methods whereby a superplastic metal blank clamped adjacent a female or male forming surface is heated to a temperature suitable to place the blank in a superplastic state. A uniform fluid pressure, such as air for example, is applied at an appropriate rate against the surface to press or force the sheet into contact with the forming surface. A pressure differential across the sheet is necessary to generate the required air pressure, which can be provided by creating a vacuum or directing air pressurized at a value higher than atmospheric against the sheet.
A number of advantages of superplastic forming have been noted. Severe forming can be accomplished in a single step rather than several steps. In addition, if one or more separate steps are required to form a shape by a conventional forming method, one or more intermediate anneals might also be required, which superplastic forming can eliminate. Tooling costs are relatively low since usually only a single male or female die is required, as compared with matched die sets or' multiple tools, which may be required with conventionally forming. Tooling costs are also usually lower because deformation forces required to effect superplastic forming are substantially less than required for conventional forming.
On the other hand, superplastic forming is often slower than forming conventionally. Another problem encountered in superplastic forming certain types of shapes is nonuniform thinning as the part is being formed. In superplastic forming, only unsupported metal is deformed; that is, metal' which is not touching the die surface. When superplastic forming a relatively deep shape in a female die, for example, there can be a very substantial difference in thickness between the metal in the sidewalls that along the bottom, and, more particularly, the metal in the corners.
Laycock et al U.S. Pat. No. 4,045,986 proposes a solution to the problem of nonuniform thinning. In a first step of the proposed solution, a suitably heated blank is firmly clamped around its periphery. A first uniform' fluid pressure is then applied against the sheet at an appropriate rate to force it into contact with a female mold which has a shape intermediate the final desired shape. In an alternative, the sheet may be forced against a flat surface spaced away from the plane of the clamped blank periphery. The flat surface may be stationary or advanced in opposition to the fluid pressure. This first forming step provides a preform for further forming around a male mold. After making the preform, application of the fluid pressure is stopped and the female mold on the flat surface is withdrawn. Then a male mold contoured to the final desired shape is advanced against the preform from the side opposing the female mold until the preform is in contact with a substantial area of the male mold. A reverse uniform fluid pressure opposite from the first pressure is then applied against the sheet as the male mold continues to advance until the preform is completely shaped around the male mold. Timing of the application of this reverse pressure, variations in the pressure differential, and speed of advance of the male mold may be adapted to suit particular requirements. According to the patent disclosure, this method minimizes nonuniform thinning which might otherwise occur in superplastic forming a part.